This invention relates to processes and fabrication of memory devices, and specifically to a method of depositing ferroelectric and dielectric thin films, such as PbZrxTi1xe2x88x92xO3(PZT), SrBi2Ta2O9(SBT), Ba1xe2x88x92xSrxTiO3 (BST) and Pb5Ge3O11 (PGO), using ferroelectric and dielectric solutions in liquid-delivery metal organic chemical vapor deposition (MOCVD) techniques.
Liquid delivery MOCVD processes have been used for deposition of ferroelectric and dielectric thin films. Compared with solid source techniques, the advantages of using liquid source for MOCVD are well known and include achieving homogeneity at an atomic level in a solution, which leads to accurate control of the stoichiometry of the deposited films, which have, when using single source techniques, complex compositions. Because the chemical reactants for a liquid source may be purified conveniently by distillation and crystallization, films of high purity may be fabricated by liquid-delivery MOCVD processing. Another advantage of liquid source delivery is that liquid source materials are easy to transport from a source container to a vaporizer, and may be accurately infused into the reaction chamber using a liquid flow meter. Liquid source materials may be atomized to increase evaporation area, which will increase evaporation amount and promote the complete evaporation of the liquid source material. However, because liquid-delivery MOCVD techniques are relatively new commercial technologies, there are still many problems to be dealt with, including those related to surface roughness, surface morphologies and properties of ferroelectric and dielectric thin films deposited by the liquid-delivery MOCVD techniques. In liquid-delivery MOCVD processes, solvents are used to dissolve precursors, to transport the precursors from a source container to a vaporizer, and to promote atomization of the precursors, resulting in complete evaporation of the precursor.
Because the solvents are introduced into a MOCVD reactor, and join in the precursor reactions and film depositions, the solvents have a significant effect on the film qualities, such as surface roughness, morphology and other properties. Selection of solvents is an important decision when using MOCVD techniques for ferroelectric and dielectric thin film deposition. In spite of the importance of solvent selection, most known research has been directed towards selection of precursors, with only a minor amount of research being directed towards solvents selection for MOCVD processes.
The challenge of preparing PZT ferroelectric metal oxide thin films via MOCVD methods are described in G.-R. Bai, A. Wang, I.-F. Tsu, C. M. Foster, and O. Auciello, Integrated Ferroelectrics, Vol. 21, 291(1998); M. Shimizu, H. Fujisawa and T. Shiosaki, Integrated Ferroelectrics, Vol. 14, 69(1997); and U.S. Pat. No. 5,453,494 to Kirlin et al., for Metal complex source reagents for MOCVD, granted Sept. 26, 1995.
U.S. Pat. No. 5,820,664 to Gardiner et al., for Precursor compositions for chemical vapor deposition, and ligand exchange resistant metal-organic precursor solutions comprising same, granted Oct. 13, 1998, describes the difficulties in this area related to the various precursors which must be used.
Currently, Pb(TMH)2 is an accepted lead precursor, however, its low volatility and poor stability renders its application in MOCVD difficult, as described in I.-S. Chen, J. F. Roeder, T. E. Glassman, and T. H. Baum, Chem. Mater., 11 No. 2, 209 (1999). With the introduction of a liquid-delivery system, the delivery of precursors is greatly improved, but there are still many problems, as described in J. F. Roeder, B. A. Vaartstra, P. C. Van Buskirk, and H. R. Beratan, Mater. Res. Soc. Symp. Proc. 415, 123 (1996).
The key problems are the precursor""s stability and volatility, the solution reactions in PZT precursor solutions, the precursor""s solubility, the precursor""s decomposition tendency in the heated delivery line, the purity of the precursor solutions, and the block of the delivery line and the vaporizer. To solve these problems, the use of different precursors is necessary. The existing precursor solution is the combination of tetrahydrofunan (TBF) and tetraglyme and iso-propanol, in which the solid ferroelectric precursors are dissolved.
A ferroelectric and dielectric source solution for use in chemical vapor deposition processes includes ferroelectric and dielectric chemical vapor deposition precursors; and a solvent for carrying the ferroelectric/dielectric chemical vapor deposition precursors taken from the group of solvents consisting essentially of type A solvents, including tetraglyme, triglyme, triethylenetetramine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine; N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-pentamethyldiethylenetriamine; and 2,2xe2x80x2-bipyridine; type B solvents including tetrahydrofuran, butyl ethyl ether, tert-butyl ethyl ether, butyl ether, and pentyl ether; and type C solvents including iso-propanol, 2-butanol, 2-ethyl-1-hexanol, 2-pentanol, toluene, xylene and butyl acetate; and mixtures of solvent types A, B and C.
An object of the invention is to develop optimum liquid-delivery MOCVD processes for ferroelectric and dielectric thin film deposition using mixed solvents.
Another object of the invention is to improve the surface roughness, surface morphology and ferroelectric/dielectric properties of the deposited film using the mixed solvents.
A further object of the invention is to provide a precursor solution which is stable, i.e., does not precipitate, when stored for an extended period of time.
Another object of the invention is to provide a solution having metal xcex2-diketone solid precursors dissolved in a precursor solution wherein the metal concentration remains nearly constant during extended storage.
Yet another object of the invention is to provide a precursor solution which yields deposited metal oxide ferroelectric and dielectric thin films having good ferroelectric and dielectric properties, respectively.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.